Lecture 22 Air-Mass Thunderstorms (CH18)faculty.fiu.edu/~hajian/MET4300/MET4300_SWX_LEC22.pdf · 2019-02-27 · Four lectures for CH18 • Airmass thunderstorms (Today lec 22) •

MET 4300

Lecture 22Air-Mass Thunderstorms (CH18)

Thunderstorms• Also called cumulonimbus clouds.

Thunderstorms are tall vertically developing clouds that produce lightning and thunder.

• Most are not severe.• NWS’s severe thunderstorm

definition:1) large hail (diameter>= 1 inch; was ¾ inch before Jan 2010)2) wind >= 50 kt3) a tornado

• Lightning occurs in all thunderstorms, so it is not used a criterion for sever thunderstorm/ weather.

Locations of Severe Weather Events in 2009

• Damage from severe thunderstorms (lightning, hail, severe winds, tornadoes): $2.4 billion in US over 2000-2009. 116 fatalities and 1395 injuries annually.

Four Factors for the Formation of Severe Thunderstorms

• 1. A source of moisture• 2. A conditionally unstable

atmosphere• 3. A mechanism to trigger the

updraft, either through lifting or heating of the surface

• 4. Vertical wind shear• Large conditional instability and

strong vertical wind shear favor destructive thunderstorms

Types of Thunderstorms• Airmass (ordinary) thunderstorms: single isolated

thunderstorm that forms in absence of vertical wind shear• Frontal squall lines: a very long line of thunderstorms

developing along frontal boundaries during the cool season.• Mesoscale Convetive Systems (MCS): develop along

subtle boundaries (differential heating of air over surfaces with different properties) in warm season, self-organized, in a scale of a few km to several hundred km

• Supercell thunderstorms: violent storms developing if winds increase rapidly with height ahead of a strong front or a less distinct boundary. Very strong, rotating cells that produce most violent weather and the majority of large tornadoes. Can occur in both warm and cool season.

Four lectures for CH18

• Airmass thunderstorms (Today lec 22)

• Mesoscale Convective Systems (MCS, Thursday Lec. 23):

• Frontal squall lines: Next week Lec. 24

• Supercell thunderstorms: next week, Lec.25

Airmass Thunderstorms (TR)• Airmass TR occur well

within an airmass rather than along a front.

• All convection requires:– Conditional instability– Lift or heating– Moisture

• Airmass TR have– One updraft <10 m s-1

– A few km across– Extends through the depth of

the troposphere– Low shear– Moderate instability CAPE <

1000– A well defined life cycle: 1 h – Isolated single cells– Move nearly with the

surrounding wind

Airmass Thunderstorms• Often form along weak

boundaries, and can be triggered by surface heating or lifting along mountain

• Airmass TRs are found far from frontal boundaries (where wind shear is strong)

• Precipitation size could cover 15-20 km in late stage

• In absence of wind shear, they grow vertically without any significant tilt

Soundings and the Structure of Convective Clouds

Life Cycle of Airmass Thunderstorms

Cumulus Stage• Clouds consist of a warm

buoyant plume of rising air– updraft

• The updraft velocity increases rapidly with height.

• Clouds are composed primarily of small liquid cloud droplets, with little or no raindrops or ice crystals

• Ice crystals begin to form when clouds reach -10oC to -20 oC.

• Eventually these particles grow large enough to precipitate.

Mature Stage• As the clouds reach the

tropopause, it’s in mature stage• Strong stability of the

stratosphere quickly inhibits further ascent

• Updraft in most TRs are strong enough that the clouds will extend a short distance into the stratosphere, producing a bulge at the cloud top, calling an overshooting top

• Downdraft created by precipitation, enhanced by evaporation cooling due to entrainment.

• A cold pool forms near surface by the accumulated rain-cooled air.

• Downdraft counteracts updraft.

Dissipation Stage• At this stage, downdrafts

completely shut off the updrafts, the storm is composed of downdrafts only.

• Heavy rain and rain-cooled air descend from the storm base.

• Produce an outflow of cool air à gust front.

• New clouds will form over gust front as warm air lifted by cold poolà shelf clouds or roll clouds.

• New cells may develop near the cold air’s leading edge. May develop to MCSs.

• Air from the updraft exhausts horizontally , forming an anvil(cirrostratus, altostratus, and cirrus). Anvil is symmetric with little shear, but asymmetric if under high shear.

• Mammatus clouds form at the base of anvils (bag like) due to descending of evaporation-cooled air.

A few Important Terms Associated with Thunderstorms

• Overshooting top: a bulge at the thunderstorm cloud top when the clouds extend a short distance into the stratosphere under strong updrafts.

• Anvil: Air from the updraft exhausts horizontally at the tropopause, forming a cloud feature called anvil. Anvil is symmetric when shear is little, but asymmetric when shear is strong. Cirrus farther from the storm, cirrostratus & altostratus closer to the storm.

• Mamatus clouds form at the base of anvils, rounded “bag” hanging from the anvil, forming as particles evaporate. Evaporationally cooled air descends downward.

• Entrainment: Precipitation encounters dry air mixing in from the side and the top of the cloud.

New cell formation

In Class Activity

• Read CH 18For Next Time

• Ex. 17.1, 17.2, & 17.3

Kinds of Precipitation• Rain: Liquid water drops > 0.5 mm in diameter falling

from the sky• Drizzle: Liquid water drops < 0.5 mm in diameter falling

from the sky• Virga: Like rain but evaporates before it reaches the

ground. – Virga is a hydrometeor, but it isn’t precipitation.

• Hail: More or less spherical ice > 5 mm in diameter• Graupel: More or less spherical ice < 5 mm in diameter• Snow: Ice crystal deposited directly from vapor, or

aggregates (clumps) of crystals• Note that the boundary between Hail and Graupel is 10

times larger than the boundary between Rain and Drizzle

Terminal Velocity• Hyrdometeors fall speed is

a balance between friction and gravity.– Cloud particles: mm or cm s-1

– Rain up to 8 m s-1

– Snow 1-2 m s-1

– Hail > 10 m s-1

• VT increases for– Lower air density– Hydrometeor size– Smoother hydrometeors

What Happens in a Cloud?• Updraft leads to cooling and

condensation.• Forms cloud droplets, 10-50 μm in

diameter.• Cloud droplets collide and

coalesce to form rain• Are also carried upward to

temperatures < 0oC• Many eventually freeze• Ice grows faster than supercooled

liquid H2O• Forms snow, graupel, and hail • Which may melt to form rain if it

falls below 0oC level• Precipitation loading and

evaporative cooling force downdrafts.

What are clouds made of?

• Not of raindrops, but of suspended cloud droplets---about 100 times smaller than raindrops, i.e. 10-80 µm (0.01-0.08 mm) in diameter.

• Let’s start with cloud droplet formation in supersaturated air through Condensation of water vapor:

• Homogeneous nucleation: Water drops (or ice crystals) might form in pure air with nothing but dry air and H2O involved.

• Each water molecule added has to do work against the surface tension. • Thus, it is hard to make cloud droplets from scratch, because the amount of

work per molecule is more when the droplet is smaller.• Homogeneous nucleation requires > 110% relative humidity (or 10%

supersaturation), which generally does not occur in the atmosphere.

Heterogeneous nucleation• Water drops (or ice crystals) form from the vapor on

some sort of substrate or by soaking into a salt particle.• For wetable nuclei, water attaches to the surface.• For hygroscopic (tending to absorb moisture from the air)

nuclei, water soaks into the surface and may actually dissolve the nucleus.

• What makes a good Cloud Condensation Nucleus(CCN)?– Reasonably big (for example, an aerosol particle) – Easily wetable (0.1 µm) or soluble (0.01 µm).

• Over land, 100-1000 CCN cm-3 typically become active at 1% supersaturation

• Over the oceans, <100 CCN cm-3 typically become active at 1% supersaturation

Growth of Cloud Droplets by Diffusion

• The cloud becomes super-saturated • Water vapor diffuses from high vapor

density to low vapor density • Rate of mass growth is proportional

– Droplet area – Supersaturation

• Rate of growth of radius– Area is proportional to radius

squared – Volume is proportional to radius

cubed – Radius growth is inversely

proportional to radius itself. – Thus there is a well-defined limit

of 10-80 μm for diffusional growth

Growth of Warm Rain• Warm Rain: Falls from clouds

where T > 0oC throughout• Autoconversion: A few

droplets collide randomly and merge, becoming larger than the others

• Hydrometeors fall near their terminal velocity

• Larger, denser or rounder hydrometeors fall faster

• In clouds warmer than 0oC, precipitation forms when larger particles sweep up smaller ones as they fall through the cloud.

• Called Collision Coalescence.

Mixed-Phase, Clouds, T<0oC• Both freezing and condensation

generally require nuclei to start the process

• Natural clouds have plenty of condensation nuclei (100,000/liter) but few freezing nuclei (10-100/liter)

• Consequently many clouds colder than 0oC contain Supercooled liquid H20 that has not yet frozen.

• May be as cold as -20oC• When some ice does form,

– the ice crystals grow – at the expense of supercooled

water droplets– Bergeron-Findeisen process

• Artificial freezing nuclei can promote growth of snow that melts to form rain

Riming • Starts with a vapor-

grown ice crystal• Falling through a

supercooled cloud• Droplets freeze on

leading edges. • Forming spongy ice• This is how graupel

forms.

Wet Growth or Wet Riming• Temperatures > -5oC• Latent heat released

by freezing keeps hailstone at 0oC.

• Coated with liquid water– Some freezes– Some is shed as drops

at 0oC

Structure of Hailstones• Updrafts in Cb’s are often

stronger than hydrometeor terminal velocities.

• Leads to recycling of graupel and hail

• Forming multiple layers of clear and spongy ice

• In very strong updrafts hailstones can get really big---

• Coffeyville KS, 22JUN03– 18 cm diameter, – 750 gm

Two episodes of “dry” riming plus initial growthThree episodes of wet riming including the last stage of growth

Aggregation• Falling frozen

hydrometeor collects other frozen hydrometeors

• Most snowflakes are aggregates of vapor-grown crystals

• Works best at temperatures > -5oC

• Big, soggy flakes vs. diamond dust

Precipitation Formation

Summary• Review cloud geometry in relation to LCL, LFC & EL• Air Mass Thunderstorms require low shear and moderate instability• Life cycle

– Cumulus---all updraft, no ice of precipitation – Mature---Mixed up & downdrafts, some ice and precipitation– Dissipating---Predominantly downdrafts, glaciated precipitating hard

• Precipitation:– Drizzle---liquid < 0.5 mm diameter– Rain---liquid > 0.5 mm diameter– Graupel---ice < 5 mm diameter – Ice---Ice . 5 mm diameter– Snow---ice crystals

• Terminal velocity---balance between gravity & friction• Hydrometeor growth by diffusion is limited to 10-80 μm• Warm rain grows by autoconversion and collision coalescence• Bergeron-Findeisen process---Ice scavenges vapor from

supercooled liquid• Hail and graupel grow by (wet & dry) riming and aggregation• In strong updrafts hail can recycle, becoming layered